1. Field of the Invention
The present invention relates to a joined metal member comprising two metal members joined with each other by press and heat generated on passing an electrical current through the metal members, and a method of joining said member.
2. Prior Art
Various methods are known for joining metal members in such cases as joining a valve seat of an engine cylinder head onto the periphery of an opening of air intake or exhaust port of the cylinder head body.
For example, Japanese Patent Publication JP-A 8-100701 discloses a method of joining a valve seat with an Al-based cylinder head body by brazing using an Al--Zn brazing material and a fluoride flux.
Also such a method is known as disclosed in Japanese Patent Publication JP-A 58-13481, where metal members are joined by resistance welding which utilizes the heat generated by contact resistance between mating surfaces of the two members. With regard to the resistance welding, it has been proposed to increase the amount of heat generated in the mating surfaces by infiltrating pores in a valve seat made of a sintered material with an infiltration metal, thereby decreasing the amount of heat generated inside the sintered material as disclosed in Japanese Patent Publication JP-A 6-58116, or to form a film on the valve seat surface and fuse the film when welding the valve seat with the cylinder head body as disclosed in Japanese Patent Publication JP-A 8-270499.
Further, as disclosed in Japanese Patent Publication JP-A 8-200148, it is proposed to make solid-phase diffusion joining (pressure welding) of the valve seat and the cylinder head body while forming a plastic-deformed layer on the joining surface of the cylinder head body without forming a molten reaction layer
When metal members are joined by press and heat generated on passing an electrical current through the metal members as in the resistance welding or solid-phase diffusion joining process, in case the one of the metal members is a sintered metal such as valve seat, it is desired to infiltrate the pores inside of the sintered material with a metal having a high electrical conductivity such as Cu as proposed in Japanese Patent Publication JP-A 6-58116. This makes it possible to lower the bulk resistivity of the sintered material by means of the infiltrating metal and inhibit the heat generation inside the sintered material when passing a current, thereby effectively generating heat in the mating surfaces and achieving better joint.
However, impregnation of an infiltrating metal into the sintered material requires an additional step in the process, which leads to increased production cost. Also, because the infiltrating metal generally increases the thermal conductivity, a valve seat or the like to be used at a high temperature suffers such a problem that the heat dissipation efficiency becomes too high due to the combined effects of both the increased thermal conductivity and the joining effect, thus making it difficult to form an oxide film on the surface, which may lead to poor wear resistance on the valve sheet.